Electrical heat tracing systems frequently utilize mineral insulated (MI) heating cables which function as auxiliary heat sources to compensate for heat losses encountered during normal operation of plants and equipment such as pipes, tanks, foundations, etc. Typical applications for such systems include freeze protection and process temperature maintenance.
MI cables are designed to operate as a series electrical heating circuit. When used in hazardous area locations, i.e. areas defined as potentially explosive by national and international standards such as NFPA 70 (The National Electrical Code), electrical heat tracing systems must comply with an additional operational constraint which requires that the maximum surface or sheath temperature of the heating cable does not exceed a local area auto-ignition temperature (AIT). Maximum sheath temperatures often occur in sections of the heat tracing system where the heating cable becomes spaced apart from the substrate surface (such as a pipe) and is no longer in direct contact with it i.e. where the cable is no longer effectively heat sunk. Such sections are typically located where heating cables are routed over complex shapes of a heat, tracing system. With respect to the heat tracing of pipes, this occurs in areas around flanges, valves and bends, for example, of a piping system.
Frequently, a heat tracing system designer is not able to utilize a single run or pass of cable for a particular installation since the higher wattage typically utilized in single runs may result in a maximum sheath temperature that exceeds the AIT. Instead, the designer will specify several lower-wattage cables operated in parallel so that the heat tracing system will operate at a low enough power density to ensure the cable sheath temperatures stay below the AIT. For example, if a piping system requires 20 watts/foot of heat tracing, the designer may have to specify two passes of 10 watt/foot cable instead of one pass of 20 watt/foot cable to keep the maximum sheath temperature of the heating cables below the AIT. In this example, the two-pass configuration will increase the cost of the installed heat tracing and can also result in configurations that are difficult to install when there is physically not enough room (such as on a small valve or pipe support) to place the multiple passes of heating cable. Thus, it would be desirable to operate a heating cable at increased power densities while reducing both the maximum sheath temperature to below the AIT and the number of passes of cable for a given application.
An approach is to use heat transfer compounds to reduce sheath temperature in electric heating cables. Heat transfer compounds have been used in the steam tracing industry to increase the heat transfer rate from steam tracers to piping. However, such compounds are only allowed in certain lower risk hazardous areas, require additional labor and material costs, and are difficult to install in non-straight sections of heat tracing, for example, around flanges, valves and bends where higher sheath temperatures are often found.
Another approach used for extreme high temperature applications in straight heating rods is to increase the surface emissivity of the heater. This increases the heater's performance by improving the efficiency of radiation heat transfer and allowing the heater to run cooler and last longer. The increase in emissivity occurs when the surface is oxidized. While increasing the emissivity can be used to decrease heating cable sheath temperatures this approach is limited since it is most effective only at very high temperatures.
A farther approach involves increasing the surface area of heating cables to improve radiation and convection heat transfer. Because of its larger surface area, a larger diameter MI cable will have a lower sheath temperature compared with a smaller diameter cable when both are operated at the same heat output (watts/foot). However, this approach increases the material costs and the stiffness of the cable.
Parallel circuit heating cables are desirable for their cut-to-length feature that is useful when installing field-run heat tracing. However, parallel heating cables employ a heating element spaced between two bus conductors and tend to be larger than their series counterparts. There are commercial non-polymeric parallel heating cables that are assembled by positioning a heating element, electrical insulation and bus conductors inside an oval-shaped flexible metal sheath or jacket. The jacket serves to house the heating element, electrical insulation and bus conductors and thus the jacket is part of the heating cable itself. In addition, the jacket protects the heating, insulating and conductor elements from impact and the environment. However, such parallel heating cables tend to be large and thus are rather stiff and their oval shape makes them difficult to bend especially in certain directions. They also have open ends and space within the cable that allows for moisture ingress that can cause electrical failure.